Process for producing acetic acid by methanol carbonylation involves multiple steps, including reaction, flashing, light-ends distillation, phase separation, heavy-ends distillation, etc. The carbonylation reaction is performed by reacting methanol and carbon monoxide in the presence of a carbonylation catalyst, a catalyst stabilizer, methyl iodide, water and methyl acetate to produce an acetic acid stream comprising the catalyst, the catalyst stabilizer, methyl iodide, methyl acetate, water, and acetic acid. The acetic acid stream is flashed to produce a vapor stream comprising acetic acid, water, methyl acetate, methyl iodide and acetaldehyde, and a liquid stream comprising the catalyst and the catalyst stabilizer. The vapor stream is subjected to light-end distillation to form a crude acetic acid product stream comprising acetic acid and water, and an overhead stream comprising methyl iodide, water, methyl acetate, and acetic acid. The overhead stream is condensed in a decanter to produce a light, aqueous phase comprising water, acetic acid, and methyl acetate, and a heavy, organic phase comprising methyl iodide and methyl acetate. The heavy, organic phase is recycled to the carbonylation reactor. Methyl iodide is a catalyst promoter of the carbonylation and is expensive. Therefore, the decanter phase separation is a crucial step for the acetic acid production process.
U.S. Pat. No. 6,677,480 teaches a method which comprises measuring the density of the decanter heavy, organic phase and using the density to adjust the feed of methanol and to regulate the temperature in the reaction zone to optimize reactor conditions. However, the control scheme disclosed in this patent cannot effectively control the decanter phase separation because the decanter is three steps downstream of the reactor. Thus any changes in the reactor conditions in response to the density measurement of the decanter heavy, organic phase cannot effectively remedy the phase separation problem in the decanter because of the delayed measurement. In summary, new methods are needed for controlling the decanter phase separation of the acetic acid production process. Ideally, the new method allows controlling the decanter phase separation by measuring the composition of the reactor mixture, predicting its effect on the decanter phase separation, and then adjusting the reactor or the decanter conditions in response to the prediction.